1. Field of the Invention
The present invention relates generally to a method for monitoring etching processes effected by ions, radicals and/or neutral particles activated in a plasma, and more particularly to a method for identifying an end point of material erosion of a layer to be etched during material etching in a plasma.
2. Description of the Related Art
The designational, or predeterminable, and dimensionally true structuring of thin, insulating layers by etching processes effected by ions, radicals and/or neutral particles activated in a plasma is a critical process step in the manufacturing of VLSI semiconductor components, as may be derived from the book "VLSI-Technology" by S. M. Sze, McGraw Hill, International Book Company (1984) in the chapter "Dry Etching", and particularly at pages 305, 306 and 307. This process is also known as dry etching.
In what is referred to as the dry etching method, a structured layer of photoresist has previously been situated on a layer to be shaped or on the substrate to be shaped. The problem then is to transfer the structure of the photoresist dimensionally accurately onto the layer of the insulating material or substrate lying therebelow. To accomplish this, the substrate coated in predetermined locations with the photoresist is bombarded with suitable ions, or is introduced into a plasma. Particles such as ions and neutrals chemically activated in the plasma react with the exposed layer or substrate material upon formation of a volatile compound. This reaction causes localized material erosion to occur. The localized material erosion can also be promoted by simple impact processes using atoms, ions, or molecules, also known as atomizing or sputtering.
Care must be exercised during the development of etching processes to ensure that the reactivity of the plasma, as determined by a suitable selection of the initial gases, is such that the erosion rate of the photoresist or of the material lying under the layer to be etched is as low as possible. This means that a separate etching process must be developed for each layer material. Furthermore, since extremely high reproduceability is demanded to the slightest dimensional tolerances, for example.+-.0.1 .mu.m, the processes must be constantly monitored. In particular, the end point of the etching for each layer must be precisely identified.
Spectrometric methods for characterizing the plasma have been mainly used up to now for identifying the etching end point (see S. M. Sze, VLSI Technology, McGraw Hill, International book Company (1984), page 334). To do so it is essential to find an emission line or absorption line of a molecule in the plasma that is characteristic for the respective etching process. The intensity of this line decreases rapidly when the material to be etched has been completely eliminated. For a known layer thickness, the etching rate is calculated from the time period running from the start of the etching up to the intensity decrease of the spectrometric emission or absorption line. Using this method, it is obvious that averaging occurs over the etching behavior of all wafers being etched in the system. Samples must be taken for local identification of the etching rate by, for example, identifying the extent of material erosion using a scanning electron microscope. This interrupts the etching process, however, and can even render the tested samples useless. Local characterization of the plasma is extremely difficult since the plasma itself is to greatly influenced by the required probes, for example, Langmuir probes. For further details, see the article by Clements in Journal of Vacuum Science Technology, 15 (2) March/April 1978, pages 193-198.
European Patent Application 0 146 720 discloses a measurement of a layer but for a layer format of a metallic layer.